Before finishing various products by performing forging, rolling or extrusion against a billet (ingot), it is necessary to soften the billet by heating it, for example, to a settling temperature 1250° C. When an attempt is made to keep a rod-shaped billet at a settling temperature by heating a single coil, as a temperature distribution becomes non-uniform, it often results in a waste caused by a failure that it does not become at a predetermined temperature in a transient time such as during a standby mode and when transitioning from a standby mode to normal heating mode. Further, when an attempt is made to keep both end portions at a settling temperature, the central portion becomes at a high temperature and the furnace itself is sometimes dissolved. Therefore, an induction heating device is used for heating, in which an induction heating coil is divided into multiple coils and a power control is performed by connecting a high-frequency power source (e.g., an inverter) to each of the divided induction heating coils individually.
However, as each of the divided induction heating coils is disposed close to each other in order to prevent a temperature between the induction heating coils from falling, mutual induction inductances M are present, thereby generating mutual induction voltages. Therefore, each of the inverters is operated in parallel via mutual inductance and it may cause a mutual power transfer between the inverters when having a mutual phase shift of electric current between the inverters. In other words, as phase shifts occur in magnetic fields among the divided induction heating coils due to a phase shift of an electric current in each of the inverters, magnetic fields in the vicinity of the boundary of the adjacent induction heating coils are weakened, thereby reducing the density of heat generated by an induction heating power. As a result, temperature variations may occur on the surface of the heated object (such as a billet and a wafer).
Hence, a technique of Zone Controlled Induction Heating (ZCIH) was proposed by inventors and others, with which technique, even under a situation that a mutual inductance M exists between the adjacent induction heating coils and causes a mutual induction voltage, by preventing a circulation current from flowing between the mutual inverters as well as preventing heat density from degrading in the vicinity of the boundary of the divided induction heating coils, it is capable to appropriately control the induction heating power. According to the ZCIH technique, each power supply unit is provided with a step-down chopper and a voltage source inverter (hereinafter referred to simply an inverter). Then, each of the power supply units divided into a plurality of power supply zones is individually connected to each of the induction heating coils, respectively, for supplying power.
In this case, the respective inverters in each of the power supply units are controlled for current synchronization (i.e., synchronization control of a current phase), and by synchronizing phases of currents flowing in the respective inverters, circulation currents are prevented from flowing mutually among the plurality of the inverters. In other words, by suppressing electric currents from flowing mutually among the plurality of the inverters, over-voltages are avoided from occurring by the regenerative electric powers flowing to the inverters. In addition, by synchronizing phases of currents flowing in the respective divided induction heating coils, a heat density by an induction heating power is intended not to be degraded rapidly in the vicinity of the boundary of each of the induction heating coils.
Furthermore, by varying an input DC voltage of each of the inverters, each of the step-down choppers controls the current amplitude of each of the inverters, thereby controlling an induction heating power supplied to each of the induction heating coils. That is, a ZCIH technique disclosed in Japanese Patent Application Publication No. 2010-287447A, by performing current amplitude control for each step-down chopper, controls a power of the induction heating coil in each zone, and by controlling synchronization of current phases of respective inverters, intends to suppress circulating currents mutually among a plurality of the inverters, and homogenizes a density of the heat generated by the induction heating power in the vicinity of the boundary of each of the induction heating coils. By the control system for the step-down chopper and the control system for the inverter performing individual controls using such a ZCIH technique, it is possible to control a heat generation distribution on the object to be heated as desired. That is, it is possible to perform a rapid and precise temperature control and a temperature distribution control, using the ZCIH technique disclosed in Japanese Patent Application Publication No. 2010-287447A.
According to a technique disclosed in Japanese Patent Application Publication No. 2010-287447A, a current resonance inverter is configured by connecting a resonant capacitor in series with a heating coil, then a single converter (chopper) is connected to a plurality of the resonance inverters as a power source for supplying a DC power thereto, wherein, by varying a power supply voltage applied commonly to the plurality of the resonance inverters and increasing a phase difference between the rising timing of the rectangular wave voltage and the zero-cross timing of the resonant current, an inverter circuit realizes a ZVS (Zero Voltage Switching) and reduces a recovery loss at a commutation diode.
Further, a technique is disclosed in Japanese Patent Application Publication No. 2004-134138A for supplying a DC power at the same time to each of inverters individually connected to each of a plurality of induction heating coils, thereby operating a plurality of the induction heating coils concurrently. By obtaining a coefficient which makes the ratio of a rated output voltage during the rated output current operation, and a sum of a rated voltage drop and a rated induced voltage, equal to or greater than a predetermined value, and a phase angle between the rated output voltage and current of the inverter to be controlled, an output frequency of an inverter to be controlled is controlled during a normal operation so as to gain the coefficient (“2” in its embodiment) and phase angle obtained above.